Having the only safety built into the trigger is realistically NOT a safety at all. What I mean is, if that is Glock's idea of a safety, then I'll argue that a trigger by itself is the ultimate safety and every gun has one! The point is, don't pull the trigger and it won't go bang! Well, DUH!
The design flaw of the Glock is that if anything enters the trigger housing, a stick, an untucked shirt, a holster edge, whatever, it can easily depress the integral trigger safety thus unintentionally making the weapon ready to fire. Futher unintentional movement then discharges the weapon and the shooter may not be aware of the condition existed and in some cases may not even be holding the pistol!
Having an active grip safety and/or manual slide safety (when used) eliminates ANY and ALL chances of foreign objects or a misplaced finger from discharging the firearm. The gun simply will not discharge even if you yank on the trigger alone. So, while Glock claims to have a safety, it is practically and effectively USELESS as a safety device regardless of the claims of the manufacturer or user.
It's like having one of those golf carts that start the engine up when you press on the gas. If you keep your foot off the gas, it won't start thus theoretically, won't go anywhere unless you are in control of it and intentionally press the gas pedal. But it's possible that a golf bag or other weight may fall on the pedal when nobody is on the cart and it goes on a drive by itself. The pedal is has a built in safety, don't push it and the cart won't start and go. Poor ideology in safety design. Turn an ignition switch to off when not riding the cart and now it's truly safe even if the gas pedal is accidently depressed.
That said, fuctionality wise, I will concede and admit the Glock is a superb piece of machinery. It's butt ugly IMHO, but it's reliable, it's accurate, it's simple and it goes bang everytime the trigger is pulled which by the way is also it's biggest most glaring problem.